1. Field of the Invention
The present invention relates to predistortion of nonlinear circuits/systems, and particularly to a predistortion circuit with concave characteristic that negates the convex distortion characteristic of a typical solid state power amplifier.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM), qaudri-phase shift keying (QPSK), and multicarrier code division multiple access (MC-CDMA) are widely used in wireless local area networks (WLAN). This is attributed to one or more of the following attractive features: large dynamic signal range, robustness against multi-path fading and impulsive noise, efficient usage of the available bandwidth, efficient digital signal processing, robustness in case of frequency selective channels, multiple access capability and narrow-band interference rejection. However, these modulation schemes are vulnerable to the nonlinear distortion introduced by the solid-state power amplifiers (SSPAs) widely used in wireless communication systems.
In designing SSPAs designers are facing two conflicting requirements. To increase the efficiency, SSPAs are usually driven into saturation. This would result in severe nonlinear distortion usually causing a spread of the amplified spectrum (spectral regrowth), affecting adjacent channels, in addition to inband intermodulation products, affecting the channel of interest. This would degrade the overall performance of the wireless communication system incorporating SSPAs. Prediction of the nonlinear performance of SSPAs, measured by its intermodulation performance, is therefore essential in order to evaluate the communication system performance. The availability of a mathematical model, for the transfer function, is a prerequisite for the prediction of the nonlinear performance of SSPAs. Inspection of the available literature reveals that a Cann model is widely used for modeling the transfer function of the SSPAs yet the Cann model can't predict the nonlinear performance observed in SSPAs operating at relatively low power levels. Moreover, the Cann model can't predict the sweet point; where a notch is observed in the intermodulation performance, of the SSPAs operating near saturation
Thus, a technique for predicting the intermodulation performance of the SSPA; without recourse to complicated measurements, that can lead to the design of a concave characteristic solving the aforementioned nonlinearity problem is desired.